Overcurrent sense circuit

ABSTRACT

An overcurrent sense circuit for sensing an overcurrent condition at the output of a power supply uses inherent D. C. resistance and the voltage drop associated with current flow through that resistance to generate a logic signal indicating the overcurrent condition. A current source offsets the maximum allowable voltage drop across the equivalent resistance for normal operation. The offset voltage and voltage drop across the equivalent resistance are fed into a comparator. When the voltage drop across the equivalent resistance exceeds the maximum allowable voltage drop due to an overcurrent condition, the comparator switches and thereby provides an output indicating the overcurrent condition.

United States Patent [1 1 Clark et al.

lll] 3,919,565

i 1 Nov. 11, 1975 i 1 OVERCURRENT SENSE CIRCUIT 3.295.052 tz/wos Martin 3H7/235 x [75] inventors: Virgil R. Clark. Stewartville: Carroll D, G d R k; Ten-an Primanv Examiner-John Zazn'orsky W, K R h B Attorney. Agent. or Firm-Donald F. Voss Price. Byron. all of Minn. [73] Assigncc: lnternatlonal Business Machines ABSTRACT l 9 An overcurrent sense circuit for sensing an overcur- 33 A 3, 1974 rent condition at the output of a power supply uses in- 7 herent D. C. resistance and the voltage drop associpp No.1 458538 ated with current flow through that resistance to generate a logic signal indicating the overcurrent condi- 53 5 (3| 307/235 307/202; 340/253 A; tion. A current source offsets the maximum allowable 340/253 P voltage drop across the equivalent resistance for nor [51] Int "03K 5/20; H02 7/20v mal operation. The offset voltage and voltage drop 5 Field f Seal-chm" 307/202 235 340/253 A across the equivalent resistance are fed into a compar- 340/253 ator. When the voltage drop across the equivalent rev sistance exceeds the maximum allowable voltage drop 56] References cited due. to an overcurrent condition. the comparator switches and thereby provides an output indicating the UMTED ES N S overcurrent condition. 3.( 7t1.l35 l/I963 Farnsworth et al 307/235 X 3.l73.()78 3/l965 Farnsu'orth 307/202 X 7 Claims, 2 Drawing Figures OVERCURRENT m SENSE OUTPUT I 20 POWER CURRENT s pp GENERATOR US. Patent. Nov. 11, 1975 3,919,565

OVERCURRENT 10 SENSE OUTPUT 20 POWER 1 1 CURRENT SUPPLY RL GENERATOR OVERCURRENT w W 0] 1. SENSE OUTPUT Q 1 W T R20 'VW RHIOK) POWER SUPPLY l OVERCURRENT sense CIRCUIT BACKGROUND OF THE INVENTION user to re-try anoperation, because it does not cause an interruption which requires maintenance attention such as required by a blown fuse.

2. Description of the Prior Art Traditionally it has been the practice to protect a,

power supply and utilization system from an overcur-....

rent condition by fuses, overload circuit breakers and other like devices. These devices are generally adequate for protection purposes but they usually'do not permit automatic restoration of the power-supply.'ln the present invention, the overcurrent condition y is sensed in a manner to permit automatic restoration of the power supply. The present-invention also makes-it easy to diagnose the source of the overcurrent condition.

SUMMARY OF THE INVENTION The principal objects of the invention are to provide an improved overcurrent sense circuit which: i

a. enables automatic recovery from the overcurrent condition;

b. is sensitive to extremely small lions;

c. is insensitive to power supply voltage variations, d. is insensitive to high frequency noise, and c. is easily adjustable.

overcurrent condipower supply and it provides a voltage drop indicative of the amount of load current. In one instance. an in ductor in the power supply was used for the impedance REQ. In another instance, a piece of resistance wire was inserted in series with the output line of the power supply. A resistance wire having a resistance of 0.012 ohms was used in a power supply having an output voltage of i2 volts..

An offset voltage is generated by current generator I5 and resistor R0 so as to set a maximum allowable voltage drop across resistor REQ for normal operation. The offset voltage makes the high voltage side of rcsistor REQ to look low to comparator 20. When an overcurrent condition exists, the high voltage side seen by the comparator remains at the same voltage level as set by the current source. however, the low voltage side of resistor REQ goes lower because of the greater voltage across REQ and the output of comparator 20 switches to indicate the overcurrent condition. It should he noted that if there is not a true overcurrent condition such as when thcpower supply line voltage rises, comparator. 20 will not switch to indicate an overcurrent condition. This is because as the power supply voltage rises and there is no change in load current, the voltage dropacross resistor REQ remains the same and there is no differential to cause the comparator 20 to switch.

. with resistor R1. Resistors R3 and R4 and capacitor C1 The foregoing objects'are achieved by detecting a voltage drop across the inherent'D.C ..resistance in the power supply lineg reater'than'a' predetermined-offset 1 voltage generated from a current source overcurrent condition. 1 7

By varying the current from the'current source, the

maximum tolerable overcurrent condition can be to indicate the provide noise suppression at high frequencies. The need for the high frequency noise suppression is that power supply 10 can be a high frequency type of power supply and therefore it would be necessary to prevent highfrequency noise from switching comparator 20 and eliminate a false indication of an overcurrent condition. The combination of resistors R3, R4 and capacitorCl causes signals above lKHz to by-pass comparaage drops can be sensedby detecting when they exceed t extremely small offset voltages."

DESCRIPTION OF THE DRAWINGS FIG. I is a'schematiccircuit diagram generally illustrating the inventiont-and. y FIG. 2 is a schematic circuit diagram. illustrating a frequency noisesuppression.

DESCRIPTION- preferred embodimen tof the invention including'high i so.

pears as a short circuit and at lower frequencies it presents a very high impedance path.

So far as sensing the overcurrent condition, the circuitof FIG. 2 operates substantially the same way as the circuit of FIG. I. The base of transistor T1 is connected to ground potential and its emitter is connected to -5 volts via resistor R2 whereby transistor T1 is normally conducting and causes a current flow through resistor RI to generate' an offset voltage. This offset voltage makes the high voltage side of resistor REQ appear low to comparator 20 relative to the low voltage side of resistor REQ. However, when load RL draws a current amounting to an overcurrent the high voltage side of resistor REQ remains the same but the low voltage sidethereof goes lower. This change in voltage is seen bycomparator 20 and itsswitches to indicate the overcurrent condition.

" Comparator 20 is a conventional differential amplifier. which has an output at one voltage level when one input is at a higher voltage level than the other input. The output of the differential amplifier switches to illlother voltage level when the other input voltage becomes greater than the one input voltage. This change in output voltage level is the signal for indicating the overcurrent condition. The current provided by transistor T] can be easily changed so as to change the maximum tolerable overcurrent condition. As a practical matter. the way to do this is to either change the value of resistor R2 and thereby change the amount of current conducted by transistor Tl or change the value of resistor Rl to change the offset voltage.

From the foregoing it is seen that the invention provides an overcurrent sense circuit which can be sensitive to extremely small overcurrent conditions. This is because the resistance REO can be made very small such as a few milliohms and similarly the offset voltage provided by the current source [5 and resistor Rl'can'.

he on the order of a few millivolts. Further it is seen that the maximum allowable overcurrent condition can be changed by merely changing the current provided by current source 15. it' is also seen-that the overcurrent sense circuit is insensitive to power supply voltage variations because the voltagejdrop across' resistor- REQ with a given current remains the same even though the power supply voltage mayvary. Further.

since no fuse is blown. the overcurrent sense circuit en-. ables automatic recovery from the overcurrent condition.

We claim: a v

1. An overcurrent sense circuit for sensing an overeurrentin a load connected across a power supply sub-- voltage independent of voltage variations occurring in said power supply. and

comparator means connected to receive said voltage drop and said offset voltage and generate one output level when said voltage drop is less than said offset voltage and another output level when said voltage drop exceeds said offset voltage.

2. The overcurrent sense circuit of claim I wherein said means for generating said voltage drop is a resistance element.

3. The overcurrent sense circuit of claim 1 wherein said means for generating said offset voltage further ineludes a resistor connected to said constant current genera- 4. The overcurrent sense circuit of claim I wherein said comparator means is an operational amplifier.

5. The overcurrentsense circuit of claim 3 wherein said-constant current generator comprises a transistor.

a voltage source.

and a resistor connected between said voltage source and said transistor.

6. The overcurrent sense circuit of claim i further comprising a high frequency noise suppression circuit connected between said comparator means and said means for generatingsaid voltage drop and said offset voltage.

7. The'overcurrent sense circuit of claim 6 wherein said high frequency noise suppression circuit comprises a pair of resistors, and

a capacitor connected across said resistors whereby said capacitor presents a low impedance path to high frequency noise signals to shunt the same from Xsaidcomparator means.

i e t i i 

1. An overcurrent sense circuit for sensing an overcurrent in a load connected across a power supply subject to voltage variations comprising: means connected between said power supply and load for generating a voltage drop proportional to current flowing through said load, means including a constant current generator connected across said power supply for generating an offset voltage equal to a predetermined maximum voltage independent of voltage variations occurring in said power supply, and comparator means connected to receive said voltage drop and said offset voltage and generate one output level when said voltage drop is less than said offset voltage and another output level when said voltage drop exceeds said offset voltage.
 2. The overcurrent sense circuit of claim 1 wherein said means for generating said voltage drop is a resistance element.
 3. The overcurrent sense circuit of claim 1 wherein said means for generating said offset voltage further includes a resistor connected to said constant current generator.
 4. The overcurrent sense circuit of claim 1 wherein said comparator means is an operational amplifier.
 5. The overcurrent sense circuit of claim 3 wherein said constant current generator comprises a transistor, a voltage source, and a resistor connected between said voltage source and said transistor.
 6. The overcurrent sense circuit of claim 1 further comprising a high frequency noise suppression circuit connected between said comparator means and said means for generating said voltage drop and said offset voltage.
 7. The overcurrent sense circuit of claim 6 wherein said high frequency noise suppression circuit comprises a pair of resistors, and a capacitor connected across said resistors whereby said capacitor presents a low impedance path to high frequency noise signals to shunt the same from said comparator means. 